The singing finger is what can mostly be referred to as the pinky. The pinky maintains the melody and it will be the decisive finger for marking and changing what the Cuatro sings. The technique referred to in the video by Cheo Hurtado, is to learn how to play with he middle, ring and pinky. This leaves the index finger open as we will typically use this finger to mark a bar whenever playing the Cuatro. So, by learning how to play with the other fingers, we can prepare ourselves for playing in positions all along the fretboard. In fact, there are times when the index finger is not even used for marking a bar as it can be used to mark individual notes as opposed to being placed across all frets.